1. Field of the Invention
The present invention relates to an arrangement for testing the transmission properties of subscriber line modules and of digital terminals of a communication system connectible thereto, and in which there is a selection control for the initialization of the respective subscriber line modules and for the connection of the individual subscriber interfaces thereof, as well as of individual terminals to a testing device.
2. Description of the Prior Art
In addition to analog telephone terminals, more and more digital telephone terminals or terminals designed for digital communication are being increasingly connected to contemporary digital communication systems. The connection of digital terminals occurs via subscriber line modules that form the periphery of the actual communication system, the jobs and operations of the subscriber line modules usually corresponding to internationally-standardized agreements. These declared standards thereby usually refer to interfaces of specific terminals with a communication system, whereby, in practice, subscriber line modules contain a multitude of such interfaces. For example, it is known to utilize a subscriber line module SLMB (Subscriber Line Module Burst System) for the connection of digital terminals that operate in a burst mode to the connection unit of a communication system, whereby this subscriber line module accepts eight of what are referred to as U200 interfaces. The information from and to the terminal are thereby transmitted according to the time separation transmission mode via a respective two-wire line in a 20 bit burst mode. The bit rate per direction amounts to 80 Kbit/s and is composed of a 64 Kbit/s voice channel and of, respectively, 8 Kbit/s for signalling and synchronization. The subscriber line module converts the exchange-side information of two pulsed-code modulation (PCM) channels from a switching matrix array of a communication system and from a High Level Data Link Control (HDLC) to the burst mode and vice-versa. A rate matching between the transmission clock and a processing clock occurs in each of the U200 interfaces, as do the four-lead, transparent conversion of the 64 Kbit/s useful channel information between the subscriber line module and the subscriber line, and, as well, the coding, overlaying or, respectively, taking and decoding of the operations signalling at the B-channel of a digital transmission. Further jobs assumed by the U200 interface are the compilation and separation of the burst information, the frame monitoring, the status control of the transmitter and time control of the adaptive receiver.
Subscriber line modules SLMD that, for example, contain four interfaces referred to U.sub.PO according to a CCITT recommendation service for the connection of ISDN terminals to digital communication systems; two-wire interfaces are thereby involved that operate according to the burst mode. Each of the four central office lines offers two B-channels having 64 Kbit/s for the message transmission and a D-channel having 16 Kbit/s for the transmission of signalling information. Here, also, the interfaces assume many-sided jobs whereof the formation of the frame structure, the encoding of the transmission data, the transmission with a transmission clock derived from the system clock, the echo compensation and the separation of the D-channel and the B-channel bit streams are referenced.
The transmission-oriented testing of the digital interfaces or, respectively, of the digital subscriber line modules, i.e. the testing of all relevant physical parameters of these interfaces, is not so satisfactorily possible, particularly in view of the multitude of transmission-oriented parameters that must be checked, that an automatic measuring or testing would be possible within the framework of the production of communication systems.